the percentages or percentage of three different groups

In a graph showing data on people using the Internet in Korea, Japan, and China, a writer wrote:

The line graph compares the percentage of people in three countries who used the Internet between 1999 and 2009.

The graph shows the percentages in three countries, so I wonder why he didn't use the plural form here.

In a different sample essay, but very similar in question style, he wrote the participation rates of ...

Why is his choice of singulars and plurals different from time to time?

P.S. Here's the link to another discussion where FF strongly disagreed with others.

• It could be careless inconsistency, or the writer noticed the error in the first, and decided not to repeat it, or it was a typing error. Commented Mar 19 at 9:20
• Thanks for your response. The thing is, I've actually seen people using both plural and singular in this kind of situation, so it's hard for non-native speakers like me to decide. :< Commented Mar 19 at 11:03
• You should note that one compares multiple things - you can't compare one percentage, so the plural is definitely required in the case of the description of the graph. You should note that 'sample essays' can be of doubtful quality. Commented Mar 19 at 11:16
• You want "The line graph compares the percentages of people in three countries who used the Internet between 1999 and 2009"? It helps to be explicit in the question. Commented Mar 19 at 12:02
• The quotation seems fine to me. Example from Pew: "the percentage of Germans, French, Spanish and Indonesians expressing positive views of the U.S. increased by at least 25 percentage points". Population Health Metrics "The estimated percentage of circumcised males in each country and territory varies considerably." I don't have a detailed grammatical explanation: "fraction" or "number" could also be used in the singular. Commented Mar 19 at 12:11

The word "percentage" should be singular in this example because it refers to a single overall statistic that represents a specific measurement or rate across those three countries.

Although the sentence speaks about a comparison between the 3 countries, which naturally means 3 separately-calculated percentages, "the percentage" is a specific measure of internet usage. Even though the comparison involves three countries, it's the same type of measure being compared in each country—each country's percentage of internet users.

Further, the word "percentage" on its own means nothing. You should view the phrase "the percentage of people" as a singular noun phrase indicating one particular kind of measurement.

From a wider context point of view, using the singular form "percentage" maintains clarity and grammatical consistency. The comparison is about how this single measure varies across the three countries. You could refer to the overall percentage of all three countries studied.

✔️ The graph shows the rod’s temperature (variable t) along its length; its endpoint temperatures are 50C and 80C (particular values t1 and t9).

✔️ The graph shows the rocket's speed (a single variable) over the past ten minutes. Comparing the rocket's speeds at two different instants (multiple values of the variable),...

❌ Comparing the rocket's speed,...
(A comparison necessarily regards multiple entities.)

The bar chart gives information about the percentages of the population living in urban areas in the world and in different continents.

Correction:
✔️ The bar chart gives information about the percentage of urban dwellers in the population (a single variable) of the world and of different continents.
✔️ The bar chart gives information about the urban-dwelling percentage of population (a single variable) in the world and on different continents.

This screenshot is phrased perfectly fine. Also, noting that “internet penetration rate”, “internet-using percentage of population” and “internet-using population percentage” are synonymous:

✔️ The graph shows three countries' internet penetration rate (a single variable) between 1999 and 2009. Comparing the penetration rates for Mexico in 1999 and 2009 (multiple values of the variable),...

❌ The graph shows three countries' internet-using percentages of population between 1999 and 2009. Comparing the population percentage for Mexico in 1999 and 2009,...

The line graph compares the percentage of people in three countries who used the Internet between 1999 and 2009.

We are comparing three countries’ trends. As the “percentage” being discussed is a single variable, it is indeed correct to be in the singular form. However, the quoted sentence's ambiguous phrasing can be improved:

✔️ The graph compares three countries' internet-using percentage of population between 1999 and 2009.
✔️✔️ The graph compares three countries' internet penetration rate between 1999 and 2009.

Finally, here's a contrived but technically correct sentence that, due to the plural form “percentages”, is ostensibly comparing multiple percentage figures, like USA 2001’s 40% and Mexico 2005’s 30%: “this graph provides a comparison of the percentages of the population who used the internet, in three countries, between 1999 and 2009.”

In a different sample essay, but very similar in question style, he wrote, “the participation rates of...”

✔️ The participation rates of Korea and Sweden are different.

• What is the difference between comparing countries and comparing percentages in your two examples? Following those two phrasings, does a writer change anything about the structure and topic of the report essay? Commented May 20 at 8:02
• Also, your answer seems to be different from Astralbee's, doesn't it? Commented May 20 at 8:04
• To me, it seems more apt to say "...the rocket's speed at different instants." The multiplicity is provided by "different instants". There's no need to pluralize speeds. Commented May 20 at 15:06
• @JackO'Flaherty "The rocket's speed at different instants" suggests a fixed speed ("the professor at the two different universities" likewise refers to one professor); since one simply cannot compare a noun phrase in the singular, "she compares the rocket's speed at different instants" doesn't make sense. On the other hand, because "various" has a more collective sense than "different", "the rocket's speed at various instants" refers to speed as a variable, so isn't implying a fixed speed. Commented May 20 at 16:22
• @anIELTSlearner I can't tell if it agrees with mine or not, I find this unreadable. Commented May 20 at 18:47